This application claims the benefit of a Japanese Patent Application No.2000-337941 filed Nov. 6, 2000, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to image forming apparatuses and storage media, and more particularly to an image forming apparatus typified by a laser printer and a digital copying machine, and to a computer-readable storage medium which stores a program for causing a computer to carry out an operation of outputting an evaluation chart (or test pattern) and/or automatically correcting a phase error between a plurality of light beams.
2. Description of the Related Art
Conventionally, there are image recording apparatuses (image forming apparatuses) which employ a multi-beam system to record images at a high speed. According to the multi-beam system, the images are recorded by scanning a photoconductive body by a plurality of light beams.
In the image recording apparatus employing the multi-beam system, it is necessary to control write timings of each of the light beams at which the images are written on the photoconductive body, so that write start positions of each of the light beams on the photoconductive body accurately match.
For example, a Japanese Laid-Open Patent Application No.56-104572 proposes a beam recording apparatus which records information on a recording medium by scanning the recording medium by a plurality of light beams. A beam detector is provided outside an effective scan region of the plurality of light beams, and a selected one of the plurality of light beams is controlled so that this selected light beam passes the beam detector in an ON state. A plurality of electrical modulating signals are generated to modulate the plurality of light beams, based on an output of the beam detector. The modulating signals are delayed and controlled depending on the arrangements of the plurality of light beams, so that recording start positions of the plurality of light beams match on the recording medium.
In addition, a Japanese Laid-Open Patent Application No.57-67375 proposes a multi-beam recording apparatus which records information on a recording medium by scanning the recording medium by a plurality of light beams. A beam detector outputs a detection signal when arrivals of the plurality of beams to predetermined positions are detected. A beam selector is provided to select one of the plurality of light beams to be supplied to the beam detector. A distributor distributes the detection signal so that recording start timings of the plurality of light beams are controlled depending on the distributed detection signal.
Moreover, a Japanese Laid-Open Patent Application No.61-137122 proposes a laser beam printer which uses a plurality of scanning laser beams. The plurality of laser beams are arranged so as not to overlap on a photodetector, and detection signals are time-divisionally and independently detected from each of the laser beams. Signal write timings are controlled depending on a correspondence of the detection signals and the laser beams.
Furthermore, a Japanese Laid-Open Patent Application No.4-35453 proposes an image forming apparatus including a plurality of light sources, a photoconductive body which is irradiated by a plurality of parallel light beams emitted from the light sources and deflected to scan the photoconductive body, light sensors disposed outside a light scan region on a main scan start side of the photoconductive body, and a pixel clock generating circuit for generating a pixel clock synchronized to synchronization detection signals which are generated by detecting the light beams by the light sensors. The number of light sensors is equal to the number of light sources. In addition, the light sources and the light sensors are respectively arranged at predetermined angles to a surface which is scanned by the light beams. The light sensors detect the corresponding light beams, so as to generate the synchronization detection signals.
The beam recording apparatus proposed in the Japanese Laid-Open Patent Application No.56-104572 is applied to cases such as when a semiconductor laser array is used as the light source and the distance between two light beams in the main scan direction on the photoconductive body, that is, the recording medium, is known. Only one specific light beam is detected by the beam detector, and the modulation signal for modulating this one specific light beam is generated based on the output of the beam detector. The output of the beam detector is delayed by a time corresponding to the distance between the two light beams, so as to generate a modulating signal for modulating another light beam. The write timings of all of the light beams are controlled in this manner.
For this reason, each light emitting position of the semiconductor laser array is positioned extremely accurately during the production process of the beam recording apparatus. However, due to inconsistencies introduced by processing errors and assembling errors of optical parts from the light source to the photoconductive body, a slight error is introduced in the optical magnification from the light source to the photoconductive body, and it is difficult to accurately match the write positions of the plurality of light beams.
On the other hand, in the multi-beam recording apparatus proposed in the Japanese Laid-Open Patent Application No.57-67375, the laser beam printer proposed in the Japanese Laid-Open Patent Application No.61-137122 and the image forming apparatus proposed in the Japanese Laid-Open Patent Application No.4-35453, a synchronization detection signal is obtained independently for each light beam, so that it is possible to more accurately control the phase of each of the light beams. In addition, even in a case where a plurality of semiconductor lasers, including laser diodes, are used as the light source, it is possible to control the write timings of each of the light beams relatively accurately.
But normally, in the multi-beam system image recording apparatus, when the semiconductor laser is used as the light source, each of the light beams are in many cases set so as to have predetermined intervals in the main scan direction in order to obtain predetermined beam intervals in the sub scan direction. Further, when a plurality of semiconductor lasers are used as the light source, each of the light beams are in many cases set so as to have predetermined intervals in the main scan direction so that the plurality of light beams independently reach the photodetector without overlap.
In addition, if a light intensity distribution of the light beam is inconsistent, it is impossible to obtain an accurate phase synchronizing signal. Moreover, if a difference exists in the wavelengths of the light beams, a magnification error is generated due to chromatic aberration of a scanning optical system which is formed by a fθ lens and the like.
In such cases, even if an accurate synchronization detection signal is obtained, a phase error, that is, a phase synchronization error, is generated among the light beams due to the magnification error. This phase error becomes larger towards a horizontal scanning end portion from a horizontal scanning start portion.
Furthermore, in the multi-beam system image recording apparatus (image forming apparatus), it is necessary to control the mount of light for each of the light beams so that output images based on each of the light beams become uniform. Normally, the amount of light is controlled for each of the light beams, based on an output of a photodiode which is provided inside a package of the semiconductor laser and detects a rearward output of the semiconductor laser. However, when using the plurality of light beams, even if the amount of light of each light beam is controlled at the light source portion including the semiconductor laser, the amount of light at the time of the exposure on the photoconductive body cannot necessarily be controlled to become uniform among each of the light beams because an optical path is different for each of the light beams. Moreover, if beam spot diameters at the time of the exposure on the photoconductive body are inconsistent, the images written by each of the light beams become inconsistent even if the amount of light are the same for each of the light beams.
In order to detect the inconsistencies of the images written by the plurality of light beams, a Japanese Laid-Open Patent Application No.11-170597 proposes an image forming apparatus which prints a dot test pattern.
However, the image forming apparatus proposed in the Japanese Laid-Open Patent Application No.11-170597 prints the dot test pattern by dots, such as 2×2 dots, having the same phase in the main scan direction. For this reason, although it is possible to detect a pitch error in the sub scan direction, there is a problem in that it is impossible to detect an error in the main scan direction.